Gravity is weird

In My Humble Opinion
61

Yes, but only for purposes of demonstrating that they’re magical. Like, Einstein famously imagined riding along a beam of light at the same speed… only to conclude that it was impossible to do so, because it led to absurdities.

Thought experiments also quite frequently posit very extreme situations, far too extreme to ever be practical, but which still follow the laws of physics as we know them. That’s fine, but it’s not magical.

We don’t need magical materials. We need carbon nanofiber. Which is something that we can produce right now. What we can’t do right now is produce it in sufficient quantities (not by many orders of magnitude). But what gives me hope for a space elevator in my lifetime is that that stuff’s enormously practical. We won’t develop a carbon nanofiber industry for building the space elevator… but we will develop it for golf clubs and fishing line and skyscrapers and suspension bridges. And then once we have the industry and carbon fiber becomes an off-the-shelf commodity, then we can build a space elevator.

62

And remember that a tether doesn’t need to be the same thickness throughout. By tapering it the weight can be reduced at the ends while having extra support in the middle, so you don’t need a material that could support a cylinder of itself for the full length.

63

Even with the intelligent tapering it’s still megatonnes of material that presently costs megadollars per kilogram. We could probably build a space staircase from a simple pile of the number of US dollar bills it would take to perfect that tether.

That’s not meant as an argument against trying now, and continuing to try diligently for decades or centuries into the future. But it is an argument against wishful thinking for prompt success.

64

Arthur C. Clarke once said that space elevators would come about 50 years after they stopped laughing.

I know about carbon nanofiber. Great stuff. One small problem. Well, not small. “many orders of magnitude” large. Except for specialized replacement purposes, nanofiber is still in the “promising” stage. Many of the same comments written in the 2000s are seen in recent years, as in this one from Wiki.

In 2014, Google X’s Rapid Evaluation R&D team began the design of a Space Elevator, eventually finding that no one had yet manufactured a perfectly formed carbon nanotube strand longer than a meter. They thus put the project in “deep freeze” and also keep tabs on any advances in the carbon nanotube field.

How many meters to geosyncronous orbit? 35,786,000. Yes, many orders of magnitude. Essentially magic. I’ve loved to be proven wrong, but I’ve been waiting on “promising” almost as long as I have for fusion power. I’m over it.

65

How would you even deploy a cable that long?

66

This is probably a better xkcd on the subject. The phrase “You could escape Deimos with a bike and a ramp” sticks in my head.

67

The only possible way to build and deploy such a cable is from geostationary orbit down (at least for an initial minimally functional cable), which means all of the millions of tons of material, equipment for fabricating the cable, et cetera, have to carried up to GEO by space launch first. This is such an obviously impractical challenge of logistics and cost that it is evident that we will not build a space elevator until there is the ability to extract and process the resources in space even if the technology existed to make high tensile strength carbon nanofiber cables thousands of kilometers in length.

And this is not withstanding all of the practical problems of operating and maintaining a ground to orbit structure such as thermal stresses, modal dynamics, repair from radiation and micro space debris damage, avoiding collisions in orbital space, and any of a large number of other practical hazards and risks. The same is true for “space fountains”, “launch loops”, and other sci-fi inspired transportation megastructures. Even where it can be asserted that they are not dependent upon near-magical materials technology, the challenges of implementing them are so far beyond the state of engineering that they are essentially fantasy.

Stranger

68

Why does a geosynchronous tether have to weigh millions of tons? I’ve seen it proposed that the first prototype could be a flat ribbon a few centimeters wide and as thin as Mylar, with a payload capacity of a few hundred pounds.

69

And how much would this flat ribbon weigh, at 36,000 kilometers long? And would it be strong enough to hold up that much weight, or does it need to be thicker to reach the point where it can self-support?

70

I don’t know where you read that but it isn’t even remotely correct. “A flat ribbon a few centimeters wide and as thin as Mylar” (say, 10 gauge or 0.3 mm) with an intrinsic tensile strength of 100 GPa wouldn’t even be able to support its own weight to GEO by several orders of magnitude, notwithstanding how such a lightweight structure would whip around with an almost infinite number of frequency modes. It isn’t even clear that a carbon nanotube cable of extensive length can be made anywhere close to the intrinsic strength of individual nanotubes, and whether you could make a length of thousands of kilometers with so few defects that you could rely upon it, even with massive redundancy, to haul large loads up to orbit.

Space elevators/beanstalks, launch loops, et cetera, are fanciful concepts that make great fodder for speculative YouTube.com videos and science fiction stories but at the detail level the mechanical properties, engineering challenges, and construction details are so vastly beyond current materials science and engineering that we cannot say when or even if they could be realized, notwithstanding the sheer volume of material and energy that would go into constructing such a megastructure. It would take massive advances in practical technologies as well as an enormous degree of automation to make a space elevator even remotely viable, and the idea that it will happen in the next few decades based upon extrapolation of existing technologies and methods is just not realistic.

Stranger

71

The way I describe escape velocity:

Imagine a motionless object in space at some altitude above the Earth. It will drop straight down.

Next, give the object some velocity parallel to Earth’s surface far below it. It will drop and hit the ground, but arch over as it falls.

Keep repeating the experiment with greater and greater speed. At some point, it’ll miss the Earth and come back around to its starting point. That’s an orbit. Your initial point is the apogee, the highest point of the orbit, and the opposite is the lowest, the perigee. The object is slowest at the apogee and fastest at the perigee.

Continue increasing the initial speed. Eventually, you’ll have a circular orbit. The speed of the object is constant, and the entire orbit is at apogee and perigee.

As we increase the speed above that, the orbit gets bigger and bigger. The orbits will take longer and longer to complete back to the starting point. The initial point becomes the perigee, and the opposite point is the apogee. The speed at the apogee becomes lower and lower as we increase the speed at the perigee, but it never becomes zero.

So what happens if we increase the initial speed so that the speed at “apogee” is exactly zero? That is the escape velocity, and the “period” of the “orbit” is infinite. As the escaping object gets farther and farther from Earth, it’s speed gets closer and closer to zero. It’s coasting to a stop.

At any initial speed above the escape velocity, the speed the object approaches is greater than zero. It’s not coasting to a stop, but will continue at that speed.

This can also be looked at geometrically. All of these motions are conic sections. The motion at the escape velocity is a parabola. The motion at less than the escape velocity is an ellipse. And the motion at greater than the escape velocity is a hyperbola.

72

Wouldn’t work: at some point the bills on the bottom of the stack would experience a compressive failure. The Hydraulic Press Channel has a number of videos in which they subject stacks of paper (ordinary 8.5x11, books, post-it notes, playing cards, etc.) to enormous compressive loads, and at some point, the stack violently explodes out from under the ram. Compilation here:

73

Yes of course. Great vid; thanks for the cite. There are lots of materials that seem at mortal scales to have huge comprehensive strength but don’t really. At least not for truly huge values of “huge”. Stacked paper being one.

My comment was intended as a metaphor for the effectively infinite cost of the project even if the many tech issues @Stranger_On_A_Train rightly points out somehow became resolved.

IOW …

However big a pile of dollars you imagine, you’re gonna need a bigger pile than that.

Should’ve included a :grin:

74

There’s a serious-looking website called the International Space Elevator Consortium that I spent some time browsing. I love technology and I love optimistic future thinking, but I have to admit that the whole site has a kind of sci-fi fantasy vibe, like something written by a bright high school student as a class assignment. Unless I missed it, there’s virtually nothing there of substance.

The space elevator idea may eventually be achieved, and I have no doubt of the incredible strength of amazing substances like single-crystal graphene. But that’s a very long way from a practical space elevator.

75

All of your objections except maybe frequency modes apply equally to the sort of massive space tower people usually envision. I wasn’t debating the intrinsic possibility of geosynchronous tethers, I was simply arguing that (provided they can be built at all) there’s no reason why the first one couldn’t be a smaller demonstration of concept.

76

Except that there is exactly such a reason. Because it needs to be strong enough to hold up its own considerable weight, there is a theoretical lower limit to how thin a space elevator cable could go. (And practically you’d probably need to go bigger than the smallest theoretically possible size).

77

I thought the strength-to-weight ratio was a fixed constant of the material being used. How is that altered by the scale of the implementation? Or iow why should making the tether thinner make it get weaker faster than it got lighter?

78

One person can produce enough static electricity to overcome the earth’s gravity and pick up a scarf or some nylon garment. On the other hand, gravity is much stronger over distance.

79

My recollection from a casual browse of the Space Elevator Consortium site I mentioned above is that the sort of “cable” envisioned by some of the concepts is more of a ribbon, around a meter wide and a couple of millimeters thick. Magical materials aside, if you’re bringing up many tonnes of material, you’d certainly need a significant surface area to grip.

80

The tensile load in the cable isn’t a constant, even approximately. As the cable goes up from the ground, it has to have sufficient tensile capability not only to maintain positive tension above but also to carry all the weight below it, hence why it is typically assumed that the “cable” will be tapered. Even at that, it just may not be possible to build a composite carbon nanofiber cable with the tensile strength to carry its own weight at any diameter even if individual nanofiber strands have an integral tensile strength in the several hundreds of GPa because of the limitations of how long it is possible to manufacture them without defects.

It is more likely that you would actually use some kind of induction drive (like a maglev turned sideways) rather than mechanically gripping the cable. However, this would basically require room temperature superconductors for both the power delivery and to drive strong electromagnets that wouldn’t lose most of their power to waste heat. So, still well beyond state-of-the-art in technology.

Stranger